The present invention relates to an image fixing method, and more particularly to an image fixing method for a full-color electrophotographic copying machine including a heating roller.
One known image fixing method in a full-color copying machine employs a heating roller and a pressing roller which are held in pressed contact with each other. According to this know image fixing method, a toner carrier (hereinafter referred to as a "copy sheet") bearing a toner image formed by superposing three or four colored toner images is heated and pressed by the nip region of the heating and pressing rollers.
An image reproduced by a full-color copying process is preferably required to have more gloss than a monochromatic image and also to have light transmission capability (color reproduciability) so that the image can be used in an overhead projector.
For the above reasons, the toners employed in the full-color copying process have small molecular weights and small molecular weight distributions, with the result that the toner cohesion will be small.
Fixing of toner to a toner sheet is regarded as adhesion of the toner to the toner sheet (see, for example, a paper entitled "Fixing ability of electrophotographic toner" at the 51st research meeting of the Electrophotographic Society.
The paper referred to above indicates that adhesion includes the elemental processes of liquefaction, flowing, wetting, and solidification, and that in order to fix toner, the viscoelasticity of toner is required to be within a certain range due to heat fusion.
In addition, the hot offset of toner (which means toner transfer when the toner is heated) is generated by cohesion destruction which occurs when the adhesive force between the toner and the heating roller is stronger than the cohesive force of the toner.
Therefore, the toner viscoelasticity and adhesion are important parameters for fixing toner.
The toner viscoelasticity is affected by the molecular weight and molecular weight distribution of toner.
FIG. 3 of the accompanying drawings shows the relationship between the molecular weight of toner and a temperature range in which the toner is fixable, the temperature range being a factor in toner fixing characteristics.
The hatched range shown in FIG. 3 which is surrounded by fixing starting temperatures and offset starting temperatures indicates a good toner fixing range.
The good toner fixing range is a stage prior to a flowing range in which the toner cohesion is destroyed, and is generally referred to as a rubber range. As is apparent from FIG. 3, if the width (temperature width) of the rubber range is narrow, the toner cohesive force tends to be small as is the case with a small toner molecular weight. If the width of the rubber range is narrow, therefore, the offset is liable to happen and a fixture failure is apt to occur.
It is known that the heating roller is made of Teflon which meets desired heat resistance and durability against a plurality of colored toners used in full-color copying.
However, even if Teflon is used, the offset is likely to take place when toner is fixed in a narrow rubber range, and hence the desired features of Teflon cannot be utilized in such a narrow rubber range.
In order to avoid the above problem, a heating roller in a fixing device in a full-color copying machine is made of silicone rubber. Silicone rubber is believed to be better in toner peelability than Teflon.
However, a toner fixing method using a heating roller of silicone rubber has the following disadvantages:
(1) Since silicone rubber is less durable against solvents and chemicals than Teflon, a fixing roller is contaminated by a pigment or dye of colored toner. As a result, the fixing roller has poor toner peelability and becomes short in service life. This phenomenon does not happen when only black toner is used.
(2) Because silicone rubber is lower in mechanical strength than Teflon, the types of copy sheets that can be used are much more limited than would be available in a monochromatic copying process, and hence a range of usable copy sheets is limited.
(3) An adhesive layer between the core of the heating roller and a silicone rubber layer around the core is susceptible to high temperatures, temperature control must be effected to give a desired level of peeling resistance to the adhesive layer on its surface held against the core. The roller surface cannot easly reach the temperature at which toner can be fixed because of the difference between heat transfer characteristics of the core and the silicone rubber layer. To solve this problem, it is proposed to position another heating roller held against the surface of the existing heating roller for keeping the surface of the silicone rubber roller at the toner fixing temperature. With this proposed arrangement, however, since the area where the heating rollers contact each other is small, heat transfer cannot well be effected therebetween, and as a consequence high-speed toner fixing cannot be performed.
(4) A greater amount of toner coheres in a full-color copying process than a monochromatic copying process. Therefore, since the thickness of the toner layer is large, surface irregularties are developed on the copy sheet even if it is heated and pressed.